An image forming apparatus employing an electrostatic electrophotography printing method generally carries out steps of charging, exposing, developing, transferring, cleaning, charge removing, and fixing, so as to form an image. In a charging step, a surface of a photoreceptor which is driven to rotate is uniformly charged by a charging device. In an exposing step, the charged surface of the photoreceptor is irradiated with a laser beam emitted from an exposure device so that an electrostatic latent image is formed. Then, in a developing step, the electrostatic latent image formed on the photoreceptor is developed with a developer by a developing device so that a toner image is formed on the surface of the photoreceptor. After that, in a transferring step, the toner image formed on the photoreceptor is transferred to a recording material by a transfer device. Then, in a fixing step, the recording material is heated and pressured by a fixing device so that the toner image is fixed on the recording material. Further, in a cleaning step, a residual toner that has net been transferred in the transferring and remains on the surface of the photoreceptor is removed by a cleaning device so as to be collected into a predetermined collection section. Then, in a charge removing step, a residual charge is removed, by a charge removing device, from the surface of the photoreceptor that has been subjected to the cleaning step. The image forming apparatus becomes then ready for formation of a next image.
The developer used to develop the electrostatic latent image formed on the photoreceptor in the developing step is generally a single-component developer made of only a toner, or a two-component developer made of a toner and a carrier. The single-component developer contains no carrier. For this reason, with the single-component developer, it is unnecessary for the developing device to employ a stirring mechanism for mixing a toner and a carrier with each other uniformly, or the like. Accordingly, the single-component developer has an advantage of a simple arrangement of the developing device. However, the single-component developer also has disadvantages, such as an inconstant charge amount of the toner. On the other hand, with the two-component developer, it is necessary for the developing device to employ the stirring mechanism for mixing a toner and a carrier with each other uniformly, or the like. For this reason, the two-component developer has a disadvantage of a complex arrangement of the developing device. However, the two-component developer is excellent in stability of a charge amount of the toner. Accordingly, the two-component developer is now suitably used in devices such as a high-speed image forming apparatus and a color image forming apparatus.
In a case where an image forming apparatus uses such a two-component developer, a circulation-type developing device is employed so as to carry the developer quickly (see Patent Literature 1). As described in Patent Literature 1, the circulation-type developing device includes (i) first and second carrying paths through which a developer provided in a developing tank (a developing container, a developer container) is carried (circulated), and (ii) developer carrying members for carrying the developer in the respective first and second carrying paths while stirring the developer, which developer carrying members are provided in the respective first and second carrying paths. According to Patent Literature 1, each of the developer carrying members is a screw including a blade having a feed screw shape and a mesh screen member. In such a circulation-type developing device, a toner is supplied into the first carrying path from a toner hopper when a toner density of the developer in the developing tank becomes less than a predetermined value.
Further, Patent Literature 2 discloses a developing device in which (i) a partition plate having a plurality of opening slits is provided between a first carrying path provided on an outer side of the developing device and a second carrying path provided on an inner side (closer to a developer carrying member) of the developing device and (ii) the plurality of opening slits are formed such that an opening slit formed at a deeper position (downstream in the first carrying path) has a larger opening area.